Have you ever seen the movie “Snakes On A Plane”? It might have occurred to you that, in real life, Burmese Pythons and Scarlet Kingsnakes don’t act like the computer-animated cobras from the film. Now imagine how a herpetologist would feel while watching it. I’m sensing some anger, some exasperation, maybe even a little righteous indignation.

Once the herpetologist works through these feelings, s/he might do an interview to capitalize on the movie’s popularity and correct some misconceptions the public has about snakes. Or, I don’t know, teach a class for non-science majors called “Snakes On The Colorado Plains”. Or write a blog post about it. That’s what I’m attempting to do here, but it’s not a movie from 2006 that’s got me thinking about reptiles. What happened was, I heard a RadioLab podcast which used the phrase “reptile brain”.

With one glance at this amazing animated gif that I found on VibeDoc.com, I am able to understand the reptilian brain.

As I’ll describe in more detail below, it’s not uncommon for neuroscience stories in the popular press to make reference to the “reptilian brain”. What does this term mean? Supposedly, it’s the most ancient part of the brain, which stuck around even as the rest of our brains evolved so we could be better than all other species. I hope my tone makes it obvious that I don’t think that’s the best description of brain evolution. I blame my righteous indignation on all the time I spent as an undergrad thinking about comparative neuroanatomy; comparative neuroscientists are very sensitive about the way people describe brain evolution. You might think the subject merits as much anger as improperly-placed apostrophes, but I hope I can convince you that that’s not the case.

The episode of RadioLab dealt with a neurodegenerative disease, frontotemporal dementia. As usual, the RadioLab team told a great story, and snuck in some interesting science at the same time. One of the hosts, Jad Abumrad, spoke with Bruce Miller, the neurologist who treated the patient described in the episode. Dr.Miller has clearly advanced treatment of neurogenerative diseases, and is an authority on frontotemporal dementia in particular (I like to research the important people that I write nasty blog posts about.) He pointed out that a “drive to repeat happens very early in the course of [the] illness”. Then he went on to outline his theory for why such obsessive-compulsive behaviors are associated with frontotemporal dementia. I don’t know for sure what Dr.Miller actually said, since Abumrad handled almost all the dumbing down of the multisyllabic science talk. Abumrad explained that, as the disease proceeds, neurons in the frontal lobes die, and they stop inhibiting other circuits in the brain. One of the areas thought to be modulated by frontal cortex is the basal ganglia, “this very ancient part of our brains” which Abumrad says you can call “our reptile brain”. If, as Miller said, the basal ganglia mainly execute “motor programs that we do repetitively every day”, then it would make sense that these repetitive actions increase as the frontal lobes stop modulating their activity.

It was at this point that Abumrad said that “birds and snakes get along with basically just this part of the brain". As someone with a background in bird brains, I reacted to this comment by emitting the following noise: "Aarrrgh!"

Hey, dudes! A recent revolution in avian neuroanatomy [1] confirms that birds have just as much basal ganglia as we do—no more, no less. They don’t have as much neocortex—the wrinkly, layered neural tissue that we think makes us so smart—but there’s still plenty of brain sitting on top of the avian basal ganglia. And they can do plenty with all that extra brain. Comparative cognition has made a cottage industry out of showing that birds are in many ways just as smart, and in some ways smarter, than we are [2]. They use tools! They recognize themselves in mirrors (unlike your stupid cat)! They can differentiate individual grad students (unlike some professors)! And songbirds—which are perhaps the most successful group, evolutionarily speaking—learn their songs from tutors, much like we learn language from our parents. Let’s see your dog do that.

(I'm not sure if Snowball the cockatoo's appreciation of the Backstreet Boys provides evidence for or against the theory that bird brains are basically just basal ganglia.)

Please don't think I'm attacking Bruce Miller and Jad Abumrad, though. What I am actually attacking is the Triune Brain Theory. When I hear someone use the phrase “reptilian brain”, I am almost positive that somewhere, somehow, the Triune Brain Theory got involved. It's a great theory. Basically, it says that the human brain is just a snake brain, with a mouse brain on top of that, and then on top of all that, the magical new brain parts that make us human, and thus superior to all other species.

The Triune Brain Theory, as proposed by Paul MacLean

The Triune Brain theory has also been shown to be completely wrong by comparative neuroanatomists [3]. And yet people continue to repeat it, probably because it aligns so well with our intuitions about what sets us apart from other animals.

To understand why the Triune Brain theory is flawed, let's consider the reptilian brain. Here's two reasons why it does not exist: (1) Reptile brains follow the same basic “floor plan” as ours. In fact, the brains of almost all vertebrates follow this same layout. Lizard cortex might not have six sexy layers like ours does, but it's definitely there, looking all layered and stuff, and sitting right at the top of the brain like ours [4]. (And data from molecular biology strongly supports the idea that, across species, cortex arises from the same tissue during development [5]). And anyways, (2) the basal ganglia were around long before any Komodo dragons were lounging on Australian shores during the Pleistocene (i.e., before animals left the water.) In fact, we now know that pretty much everything we can identify in our own basal ganglia can be found in the basal ganglia of the sole surviving species of one the most ancient vertebrate lineages, the lamprey [6].

I can't see anyone blaming addiction or frontotemporal dementia on a takeover of our behavior by "our lamprey brain" any time soon though. It just doesn't have the same ring to it. Apparently, we want to believe that, deep down, we are snakes.

Again, I’m not arguing that theories about obsessive-compulsive disorder or addiction are mistaken. Years of research show that these disorders do involve the basal ganglia. I’m arguing that, by relying on a disproven theory of evolution to explain how the brain works, those who act as interpreters of neuroscience are doing a disservice to the communities they serve. If you are a neuroscientist, doctor, or journalist, and you find yourself thinking that birds and snakes represent a shorthand way to explain brain function, please think again. There's no reason why we can't make the real story just as compelling. Our brains are not as different from other vertebrates as we might at first think. That means that, yes, we actually can learn a lot by studying the brains of other species, in spite of what some animal rights activists might tell you. (We’ve also learned much by studying invertebrates, but that’s the subject of another blog post.) There are also important differences we should keep in mind. For example, rats have a six-layered neocortex, just like us, but they don't have much in the way of pre-frontal cortex [7], the region that we think governs planning and other complex cognitive processes. These differences, seen in the context of the many similarities in brains across species, imply that we stand a good chance of figuring out what sets us apart from other animals.

There are plenty of other examples of how our current view of brain evolution gets lost in translation to the popular press, but I'll have to stop here. I also realize that ethical issues arise when we acknowledge that the brains of other species are so similar to our own, and I’d like to discuss those in a future post. Right now, though, I have to lecture a complete stranger sitting next to me that just called someone a “bird brain”.

Comments

I love the smell of righteous indignation in the morning! Thanks for the post, David. Facts aside, we still need a good way to explain the basal ganglia to DJs and middleschoolers. If not a brain that actually belongs to a reptile or lamprey, what metaphor would be less dangerous (but as simple)? For illustrations like this accuracy is the goal, but if it isn't memorable you've lost them. I'm no basal ganglia expert so I don't have much to offer, but I'm curious what others might come up with. Thoughts?

PS- I imagine when a bird researcher screams "Aarrrgh!" it sounds like Snowball singing along to the Backstreet Boys.

Hey Riley--good question. What's a simple metaphor to explain what the basal ganglia do? I'm definitely not a basal ganglia expert either--although there are plenty of them at Emory--but I think Ann Graybiel's idea of the "habitual brain" works pretty well. The whole idea of the "reward circuit" seems to capture people's imagination, too, although that takes some of the focus off the major role that the basal ganglia play in movement. Our own Darryl Neill likes to talk about the "zombification" of people as they get older, because their basal ganglia take over so many behaviors. I don't know if I'm ready for a "zombie brain" meme. (Wasn't there already a zombie-brain meme?) Anyways, like I said, I'm not trying to argue against well-developed theories of the function of the basal ganglia. I just don't think we should equate addicts with lizards and ostriches, or vice versa.

Thanks, uh, Derek. I mean, Tadd. I agree with you that it's time to bring an end to the elitism. I mean, most of us have stopped believing we're at the center of the universe. Why should we think that evolution was only leading up to us? Birds evolved after mammals, anyways.